xdg-decoration: Fix initial toplevel decoration mode negotiation (#6308)

Clients using zxdg_decoration_manager_v1::get_toplevel_decoration may
expect a receiving a zxdg_toplevel_decoration_v1::configure event to
determine the initial decoration mode, without having to go through a
zxdg_toplevel_decoration_v1::set_mode request. Hyprland was not sending
this event, resulting in unwanted decorations being drawn.

Specifically, clients using libdecor, e.g. applications using recent
GLFW, would draw GTK decorations with artefacts. This change fixes
these.

src/protocols/XDGDecoration.cpp

@@ -26,6 +26,8 @@ CXDGDecoration::CXDGDecoration(SP<CZxdgToplevelDecorationV1> resource_, wl_resou
         LOGM(LOG, "unsetMode. Sending MODE_SERVER_SIDE.");
         resource->sendConfigure(ZXDG_TOPLEVEL_DECORATION_V1_MODE_SERVER_SIDE);
     });
+
+    resource->sendConfigure(ZXDG_TOPLEVEL_DECORATION_V1_MODE_SERVER_SIDE);
 }
 
 bool CXDGDecoration::good() {